In alternating-current (AC) power distribution systems, line-frequency transformers are often use to step down the voltage. Increasingly, renewable sources of energy, such as wind turbines, solar farms, etc., are connected to the distribution network, so as to allow the transfer of large blocks of power to and from the network. Interest in energy storage systems, where excess available electrical power can be stored for later use, is also growing. In any of these applications, line-frequency transformers may be needed to step down a medium voltage (MV), e.g., between 1 kV and 72.5 kV, to a lower voltage supported by a grid-tie converter or inverter.
In these applications, the MV system may generally be assumed to be a stiff voltage source, compared with the grid-tie converter. The line-frequency transformer thus also serves to condition the voltage supplied by the grid-tie converter to that of the MV distribution.
Typical line-frequency transformers and conversion systems based on high/medium frequency transformers suitable for use in these applications are bulky, heavy, and/or expensive. In addition, line-frequency transformers built using conventional technology do not provide for dynamic control of the voltage or automatic disconnection from the source when a fault occurs. Accordingly, improved step-down AC/AC converters are needed for use in these applications.